Generally, data communication should be permitted only between authorized parties, and it is highly desirable to eliminate any possibility of unauthorized tapping of data from the system and sabotaging of the system by such unauthorized access to the system. Therefore, for some time, it has been customary to require an acknowledgment before data communication can be started between any two parties. However, conventional modem systems were not equipped with any such means of acknowledging the other parties of communication, and the computer software for data communication typically included routines for acknowledging the other parties of communication. Therefore, communication software has tended to be more complex than otherwise, and the users had to bear the cost for this added complexity of the communication software.
Furthermore, since the conventional modem systems were not provided with any means for identifying the other parties of communication, every time data was transmitted to each modem system, the mother computer of this modem system had to interrupt the current job by an escape sequence and carry out the acknowledgment sequence to find out if the data was sent from an authorized party or not. Therefore, if the incoming data was transmitted simply by mistake or for sabotaging purpose, a considerable time loss would be incurred to the receiver of this unauthorized data transmission as a result of interrupting the current job, executing an escape sequence, and recovering the status of the system.
For instance, in the case of an electronic bulletin board system, if the bulletin board system responded to every attempt of access, the system would be so overburdened by erroneous and other unauthorized attempts to get access to the system that the authorized users of the bulletin board service would have a great difficulty in receiving the service.